Optical attenuators are currently employed in optical test and measurement instrumentation and optical communication systems. The following background information is presented herein only by way of example with reference to an optical attenuator that is suitable for use in an optical fiber communication system. One type of conventional optical fiber communication system employs an optical fiber that transmits a light beam emitted by a semiconductor laser having narrow spectral linewidth characteristics. The light beam propagates through the optical fiber and strikes a light detector at an intensity or power level that occasionally oversaturates the detector and thereby causes a loss of information carried by the light beam.
Detector oversaturation can occur as a consequence of changes in the number of local area network users. Such changes cause variations in the light beam intensity received by the detector and sometimes result in information loss. To solve this problem, an optical attenuator may be used to vary the intensity of the light beam to a level within the operational range of the detector, preferably without undesirable variations in the spatial, temporal, spectral, or polarization distribution of the light beam.
Commercially available optical attenuators typically use either mechanical or nonmechanical devices to reduce the light beam intensity. One type of mechanical optical attenuator positions an object to obstruct the path of an expanded light beam. Such objects include, for example, neutral density filters or circularly graded half-silvered mirrors that are moveable or rotatable into and out of the beam path. Another type of mechanical attenuator moves a prism made of absorbing material within the beam path to vary the optical path length within the absorbing medium to increase its absorptive capacity. Because of their complexity, mechanical optical attenuators of the above-described types are costly, do not offer reliable precision control, and often increase the end separation or the transverse offset of a fiber splice. Increased end separation and transverse offset cause light loss, and may thereby cause information loss when the light beam is detected.
Nonmechanical optical attenuators may be as complex and costly as mechanical attenuators and often require additional light beam polarization control devices. One type of nonmechanical optical attenuator that includes electrooptical cells is described in Hanson "Polarization-Independent Liquid Crystal Optical Attenuator for Fiber-Optics Applications," Applied Optics, Vol. 21, No. 7, 1342 (1982). The attenuator uses a twisted nematic liquid crystal cell across which a variable voltage is applied to change the polarization state of an input beam. The cell is positioned between two planar calcite displacement prisms in an assembly that is placed in front of an optical fiber. The assembly splits the input beam into components that are either directed to or away from the optical fiber, depending on the voltage applied across the cell. The applied voltage affects the polarization states and, therefore, the directions of the beam components propagating through the cell. The attenuator is costly and very difficult to align.
One problem associated with a twisted nematic liquid crystal cell is its dependence on polarization devices, which can be a severe limitation because the state of polarization of a light beam in an optical fiber is generally unknown and often changes in response to thermal or mechanical stresses or inherent birefringence in the fiber. However, microdroplet-containing liquid crystal films offer light scattering properties that are potentially useful in an optical attenuator. A liquid crystal microdroplet film is a transparent, solid matrix containing generally spherically-shaped microdroplets of substantially pure nematic liquid crystal material. Such a film may be made, for example, from polymer-dispersed liquid crystal (PDLC) or latex microencapsulated liquid crystal material.
Whenever no voltage is applied across a PDLC film (i.e., the "OFF state"), the liquid crystal molecules randomly align and develop a mismatch between the solid polymer matrix index of refraction, n.sub.p, and the effective index of refraction of the liquid crystal microdroplets, n.sub.d, thereby strongly scattering light incident to the film. The value of n.sub.d depends on the orientation and ordering of the liquid crystal molecules in the microdroplets. Whenever a voltage of sufficient magnitude is applied across a PDLC film (i.e., the "ON state"), the liquid crystal molecules align parallel to the field and change the value of n.sub.d to approach that of n.sub.p, thereby causing the film to become transparent. The size and distribution of the microdroplets can be adjusted to change the transmissivity of the film in the OFF state, in a manner more fully described by J. W. Doane et al., "Field Controlled Light Scattering from Nematic Microdroplets," Appl. Phys. Lett. 48, 269 (1986).
The shape or geometry of microdroplets in liquid crystal microdroplet films is of interest because the shape of the microdroplets largely determines the attenuation capability of the films. For example, relaxation response time performance can be reduced and OFF state scattering characteristics can be improved for liquid crystal microdroplet films if the microdroplets are formed such that they have oblate-spheroid or disk-shape and have their maximum cross-sectional areas disposed perpendicular to the direction of light propagation. Oblate-spheroid microdroplets increase the restoring forces on central molecules to allow the cell to relax into the OFF state more rapidly. Whenever the film is in the OFF state, light will be more strongly scattered by each microdroplet because the average molecular ordering is more nearly perpendicular to the direction of light beam propagation and the effective refractive indices of the microdroplets are mismatched by a greater degree with respect to the index of refraction of the polymer matrix. The oblate-spheroid microdroplet geometry also provides liquid crystal microdroplet films with more predictable, constant optical performance over time.
The microdroplet geometry in the films can be affected by applying external pressure during film polymerization. For example, substrate surface adhesion techniques or electric or magnetic fields can be used to affect the shape of the microdroplets. Specifically, a liquid crystal film undergoes net expansion during curing because the mixture of matrix material has a higher coefficient of thermal expansion than that of a glass substrate on which the film is supported. During cooling, the film contracts only in the direction perpendicular to the glass substrate because the film adheres to the glass substrate surface and, therefore, does not contract in the direction tangential to the surface. The resulting liquid crystal film has oblate-spheroid microdroplets.
The optical performance of liquid crystal microdroplet films also may be enhanced by certain techniques carried out during the curing step of the liquid crystal microdroplet film preparation. An improperly cured liquid crystal microdroplet film can have contaminant polymer precursors solidified within the microdroplets. A post curing step generally increases the purification of the liquid crystal material in the microdroplets, thereby decreasing the number of contaminant polymer precursors and increasing the dynamic attenuation range of the film.